In addition to providing the heading in the text, the
mandatory argument of the sectioning command can appear in two
other places:

the table of contents

the running head at the top of the page

You may not want the same thing to appear in these other two
places as appears in the text heading. To handle this
situation, the sectioning commands have an optional argument
that provides the text for these other two purposes.

The title or optional title are moving
arguments so fragile commands
contained in these arguments must be protected.

All the sectioning commands have *-forms that print a title,
but do not print a number with the title and do not incement the
relevant counter and do not make an
entry in the table of contents.